Hello,
I was trying to get the "drown Noboru" achievement in last mission but I don't get prompt to throw him to water. I tried it from different angles, all possible dropping spots, different zoom, with Yuki and Aiko as well. No luck. Is this a bug? According to guides, I am standing in the right spot.

I was stuck there as well, in the end it's a matter of (tedious) pixel hunting until you find the exact spot where you can throw Noburu down, first from the building, then from the rocks.

After reading about the badge, I tried to drown him in... the well. Doesn't work. You need to go to where you start climbing the main keep - where the hookshot spot is.

It's weird because a single fall like this would be enough to kill him. When professional boxers or martial artists are knocked out, they suffer the most not from the blow itself. The real danger is hitting the ground while unconscious, or when you momentarily can't control your body. We instinctively raise our hands to protect ourselves, especially the head, or assume a safer position when falling. All defense mechanisms go down.